Downhole motor assemblies, such as mud motors, are used to supplement drilling operations by turning fluid power into mechanical torque and applying this torque to a drill bit. The drilling fluid or drilling mud is used to cool and lubricate the drill bit, carry away drilling debris, and provide a mud cake on the walls of the annulus to prevent the hole from sloughing in upon itself or from caving in all together.
One example of a drilling assembly using a mud motor is illustrated in FIG. 1. The downhole assembly includes a motor 11 which is suspended on a string of tubing in the well. Motor 11 is of a progressive cavity type, and has a tubular housing 15 that contains an elastomeric stator 17. Stator 17 is a stationary elastomeric member having cavities 19 throughout its length. A rotor 21 extends through the cavities 19, and rotates as a fluid is passed through motor 11.
The downhole assembly has a longitudinal axis 35 that coincides with the longitudinal axis of motor 11. The lower end of rotor 21 will orbit eccentrically relative to axis 35, as indicated by the numeral 37. The amount of lateral deviation from the axis 35 may be on the order of about 3.1 mm to about 6.4 mm (about ⅛ to ¼ inch), for example. Rotor 21 is connected to a connector shaft 39 by a rotor coupling 41. Rotor coupling 41 forms a rigid connection which causes the upper end of connector shaft 39 to orbit in unison with the lower end of rotor 21. The lower end of connector shaft 39 connects to a drive shaft coupling 43, which is also a rigid coupling. Drive shaft coupling 43 rotates concentrically on the longitudinal axis 35. Connector shaft 39 will flex along its length because of the orbiting movement of its upper end. The drive shaft coupling 43 is then connected via a drive shaft 45, directly or indirectly, to the drill bit.
In operation, the motor assembly will be assembled and lowered into a well on a string of tubing. Once in place, drilling mud is supplied to motor 11, causing rotor 21 to rotate eccentrically. This causes connector shaft 39 to rotate, which in turn rotates drive shaft 45 and the drill bit (not shown). Motor 11 will discharge the fluid out the lower end and thence to the drill bit for cooling of the drill bit and removal of drill cuttings, where it flows to the surface.
Drilling motors or mud motors, such as illustrated in FIG. 1, may also include a rotor catch device that provides the operator the ability to retrieve a broken motor assembly in the unlikely event of a connector separation or mechanical failure. FIG. 2 illustrates a rotor catch device 30, where like numerals represent like parts. The rotor catch device extends from the top of rotor 21 into a top sub 32. Top sub 32 and stator 15 may include threaded sections 34 to connect the two components. Top sub 32 also includes a shoulder 36. The top end of rotor catch device 30 has an outer diameter greater than the inner diameter of shoulder 36. If any part of the external body (e.g., a stator connection) breaks below the top sub, the large end of the motor catch 30 will hang up on the shoulder 36, which in turn will allow the rotor and the rest of the motor to be pulled out of the hole.